The present invention is directed to switching-mode power conversion systems. More particularly, the invention provides systems and methods for primary-side regulation with load compensation. Merely by way of example, the invention has been applied to off-line switching-mode flyback power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
Flyback converters have been used extensively for its simple structure and low cost in low-power power supplies. But in conventional flyback converters, the output-voltage regulation is often performed with secondary-side feedback, using an isolated arrangement of opto-coupler and shunt regulator (e.g., TL431). Such arrangement usually increases the system cost, size, and power consumption.
FIG. 1 is a simplified diagram showing a conventional flyback switching-mode power conversion system with secondary-side control. As shown in FIG. 1, a PWM controller 110 is used to control and drive a power MOSFET, M1, which turns on and off to control the power delivered to the load on the secondary side.
To reduce the system cost and size of the flyback switching-mode power conversion system, the converter that employs the primary-side regulation has become popular for certain applications. In the primary-side regulation, the output voltage is sensed by detecting the voltage of the auxiliary winding that is tightly coupled to the secondary winding. Since the voltage of the auxiliary winding should image the output voltage associated with the secondary winding, the detected voltage can be utilized to regulate the secondary-side output voltage. Hence, the expensive parts of opto-coupler and shunt regulator (e.g., TL431) often are no longer needed in order to save system cost and size.
FIG. 2(A) is a diagram showing a conventional flyback switching-mode power conversion system with primary-side control. The output voltage Vout is mapped to the voltage signal VINV at node INV. The adjustment of VINV often results in the regulation of Vout.
For the primary-side regulation, relationship between VINV and Vout can be expressed as follows.
                              V          INV                =                                                            n                ×                                  R                  2                                                                              R                  1                                +                                  R                  2                                                      ⁢                          (                                                V                  out                                +                                  V                                      D                    ⁢                                                                                  ⁢                    2                                                              )                                -                                                    R                2                                                              R                  1                                +                                  R                  2                                                      ⁢                          V                              D                ⁢                                                                  ⁢                1                                                                        (        1        )            
where n is the turn ratio of the auxiliary winding to the secondary winding. VD1 and VD2 are the forward voltages across diodes D1 and D2.
Setting
      k    =                            R          1                +                  R          2                            n        ×                  R          2                      ,Vout is therefore given by:
                              V          out                =                              k            ×                          V              INV                                +                                    1              n                        ⁢                          V                              D                ⁢                                                                  ⁢                1                                              -                      V                          D              ⁢                                                          ⁢              2                                                          (        2        )            
The output voltage is regulated through the regulation of the voltage of the auxiliary winding. For example, the sensed voltage, VINV, is compared with the predetermined voltage level, VREF. The difference between VINV and VREF, the error signal, is processed by the error amplifier to generate the amplified error signal. Based at least in part on the amplified error signal, the PWM/PFM signal is generated. The PWM/PFM signal controls turning on/off of the power switch thus the power delivered to the secondary side. As a result, the difference between VINV and VREF becomes smaller and smaller, and at the end, VINV should equal to VREF.
Since VINV is the image of the output voltage, Vout, the output voltage is proportional to VINV, thus VREF under certain conditions. Specifically, the output voltage is regulated at a constant level if the forward voltages across diodes D1 and D2 are constant, as shown below.
                              V          out                =                              k            ×                          V              REF                                +                                    1              n                        ⁢                          V                              D                ⁢                                                                  ⁢                1                                              -                      V                          D              ⁢                                                          ⁢              2                                                          (        3        )            
However, for a given diode, the forward voltage is current dependent; hence VD2 changes if the load current changes. In contrast, VD1 can remain almost constant because the current that flows through the diode D1 changes little when the output load current changes.
Moreover, the voltage drop across the output cable line is also proportional to the output load current. Therefore, the scheme as described above often has poor load-voltage regulation due to the voltage drops of the diode D2 and the output cable line.
Assuming resistance of the output cable line is r, we have
                              V          out                =                              k            ×                          V              REF                                +                                    1              n                        ⁢                          V                              D                ⁢                                                                  ⁢                1                                              -                      V                          D              ⁢                                                          ⁢              2                                -                                    I              o                        ×            r                                              (        4        )            
where Io is the output load current. Since different magnitudes of the load current result in different voltage drops of the diode D2 and the output cable line, the output voltage Vout is not constant at various output current levels. The output voltage Vout decreases as the output current Io increases.
FIG. 2(B) is a simplified diagram showing conventional output characteristics of a conventional flyback switching-mode power conversion system with primary-side control. As shown in FIG. 2(B), the output voltage decreases as the output load current increases. Often, the load regulation variation in such scheme is about 10% which usually cannot meet the requirements of most applications.
Therefore, it is highly desirable to improve techniques for output voltage regulation.